Here's links to some excellent resources for MS Virtual Earth development:
Virtual Earth Concepts by Mike Garza
These are some excellent concept examples for building rich user experiences with Virtual Earth. Some of the examples include: Draggable Shapes/Pushpins, Select plotted Shapes using a Selection Box, and Sidebar listing plotted Shapes that highlights the Shape on the map when hovered over. Today he just posted two new examples of adding Drag Handles to Polygons and Polylines to allow the user to easily move/resize the shapes using the Mouse. These are some really excellent example!
ViaVirtualEarth and ViaWindowsLive
Both of these sites are run by the same guys, but both contains some really excellent articles. I don't know if they are still updating ViaVirtualEarth, I believe ViaWindowsLive was created as a replacement.
Virtual Earth Interactive SDK
This is the official interactive SDK for Virtual Earth. I should need to list this since everyone probably already knows about it, but it's such a great resourec I can't leave it out.
Virtual Earth Forums on MSDN
These are the official Microsoft forums for Virtual Earth development
Virtual Earth 3D Team Blog
This is the new blog started by the Virtual Earth 3D Team. It already has a couple nice articles. I hope they keep this one up and keep adding content.
Hannes's Virtual Earth Blog
He has alot of code posted on various topics working with Virtual Earth. Definately a nice resource.
Keith Kinnan's Blog
There is alot of Virtual Earth content posted here. An excellent resource.
You can find a bunch of the articles I've written myself in the Virtual Earth category of my Blog.
If you know of any resources not listed here, please post them in a comment. Happy Mapping!
UPDATE 10/16/2008: Here's a link to another list of usefull VE resources. Also, the new VEToolkit projects goal is to simplify VE development.